HomeWhile diagnosis and treatment of individuals with tuberculosis (TB) disease is the first priority for tuberculosis prevention and care, an important second priority is identification and treatment of individuals with TB infection, but without disease. In most individuals, Mycobacterium tuberculosis (M. tuberculosis) infection is contained initially by host defenses, and infection remains silent (latent).1 However, TB infection has the potential to develop into TB disease at any time. Several risk factors, such as time since tuberculosis exposure, medical conditions, treatments or personal habits that affect host immunity, can affect an individual’s risk for progression from TB infection to TB disease.2 Identification and treatment of TB infection can substantially reduce the risk of development of TB disease3 (see Chapter 6: Tuberculosis Preventive Treatment in Adults) and thus has potential to protect the health of the individual from immediate and long-term health effects associated with TB disease, as well as protecting the public by reducing the number of potential sources of future transmission.4,5
There are 2 types of tests to identify TB infection: the tuberculin skin test (TST) and the interferon-gamma release assay (IGRA). The TST consists of an intradermal injection of a small amount of purified protein derivative (PPD) derived from a nonspecific mixture of antigens from M. tuberculosis bacteria.6 In a person who has previously been infected and developed cell-mediated immunity to these tuberculin antigens, a delayed hypersensitivity reaction will occur within 48 to 72 hours. The reaction will cause localized swelling and will be manifest as induration of the skin at the injection site.7 IGRAs are in vitro blood tests of cell-mediated immune response; they detect interferon-gamma released by a person’s T cells following stimulation by exogenous antigens specific to M. tuberculosis.8,9
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